1. Field of the Invention
This invention relates to thin glasses having a high refractive index (nd), a layer composite comprising these thin glasses, a method for the production of the thin glasses and their uses.
2. Description of Related Art
Glasses having refractive indexes in the range of above nd=1.5 up to nd=1.7 are well known. In the field of technical glasses, however, they are achieved by the addition of high amounts of lead oxide which is questionable in an ecological point of view and also disadvantageous for large economic processes. Conventional optical glasses with optical positions in the range of high refractive indexes which are used for light- and image-conducting purposes and thus are used in the conventional application fields (i.a. imaging, microscopy, medical engineering, digital projection, photolithography, optical communications engineering, optics/lighting in the automotive field) are normally prepared as a bulk material due to the geometry of the products which are subsequently produced therefrom (i.a. lenses, prisms, fibers). Ingot sections in the case of continuous ingot manufacturing, fiber core glass rods as well as optical blocks are standard formats of the production process of optical glasses. Minimum dimensions in the direction of the smallest geometric extent, normally the thickness (ingot sections) or the diameter (fiber core glass rods), of 20 mm are considered as economically and applicatively reasonable, but desirable are thicknesses of higher than 40 mm and optical blocks are only from about 150 mm.
Typically, technical glasses (prepared according to technical hot forming methods) have refractive indexes of about 1.50. Generally, glasses having refractive indexes of >1.6 are hardly suitable for technical hot forming methods, since most often they have a “steep” viscosity curve (strong change of viscosity with temperature, i.e. “short” glass properties) and most often a high tendency to crystallization. In the case of technical hot forming methods, compared to the production of ingots, these properties are a problem, since the residence time in the large technical aggregates is considerably longer and thus also the pre-nucleation/nucleation time. In addition, the technical hot forming methods are associated with longer process times and larger process windows, so that the correspondingly longer times allow the nucleuses to develop into crystals.
Therefore, there is a difference with respect to the tendency to crystallization and the steepness of the viscosity curve between the conventional optical glasses and the technical standard glasses, the physicochemical property profiles of which are specifically tailor-made for the technical basic conditions of the manufacturing aggregates of technical glasses, i.e. flat, thin and tube glasses, which are significantly larger in comparison to the manufacturing aggregates of optical glasses.
Normally, technical glasses have a “long” viscosity profile, which means that their viscosity does not vary that strongly with changing temperatures. From that, longer times of the respective single processes and also generally increased process temperatures result, which in the case of the large technical aggregates has a less distinctive negative influence onto the profitability. Furthermore, there are significantly longer residence times of the materials in the aggregates due to the flow conditions and the size of the aggregates. The residence time in the large technical aggregates is considerably longer and thus also the pre-nucleation/nucleation times are longer. In addition, the technical hot forming methods are associated with longer process times, so that the correspondingly longer times allow the nucleuses to develop into crystals. This is an extremely critical point for glasses with a high tendency to crystallization. Long glasses are advantageous in flat glass manufacturing methods, because these glasses can be processed within a larger temperature range. Thus it is not necessary that the process is geared towards a processing of the still hot glass which is conducted as quickly as possible.
If it would be considered to produce conventional optical materials in a flat glass manufacturing method (e.g. drawing, overflow fusion, down drawn, rolling), then the chemical composition of the optical glasses has to be changed in a such manner, that the content of those components must be changed (normally reduced) which are responsible for the desired optical properties of the optical glasses. Such measures would be, for example, the reduction of the proportions of TiO2, ZrO2, Nb2O5, BaO, CaO, ZnO, SrO or La2O3. This indeed results in longer glasses with reduced tendency to crystallization, but also in a considerable loss of refractive index and dispersion properties.
This is aggravated by the fact that the flat/thin glass processes which are currently preferred due to economic reasons are associated with certain chemical requirements for the glasses to be processed which are not fulfilled by the conventional optical glasses: For example, in a floating process it is not allowed for redox-sensitive components to be present in the glass. Thus, for example, standard optical components, such as the oxides of lead, bismuth, tungsten as well as the classical polyvalent refining agents (arsenic), the effective action of which is exactly based on the shift of the redox equilibrium, cannot be used.
Thus, in total there is a contradicting difference between these two conventional groups of materials, the optical glasses and the technical glasses, with respect to their processability.
There are numerous uses for thin glasses having a high refractive index besides the conventional application fields. Of course, there is the possibility to produce such thin glasses by cold reprocessing of an ingot of optical glass. But it is obvious that the cutting and polishing steps of such ingot sections are extremely expensive, and in addition they stress the glass very strongly. Thus, in this manner very low thicknesses together with large dimensions cannot be achieved. When thin glasses are mechanically polished, then this results in surface conditions which are not optimal.
GB 2,447,637 B relates to an OLED layer composite which can be used for lighting or display purposes. But here a substrate glass having a refractive index of only about 1.5 is used. The disadvantages being connected therewith have to be weakened by an antireflection layer.
US 2012/194,064 A1 describes a diffusion layer for OLEDs. The glass used there contains very much Bi2O3 and very little SiO2 and BaO. The same belongs to US 2011/287,264 A1.
Especially for the use as a substrate and/or superstrate in an OLED or a photovoltaic module it is important that between a flat glass and an adjacent layer no or only little total reflection occurs. The refractive index of the glass used should be as high as possible. Because there are many uses in layer composites in which the glass is adjacent to a layer with high refractive index, such as for example ITO in OLEDs. When the light which is generated in the OLED is emitted, then the light from the ITO layer has to enter into the superstrate of glass. The larger the difference of the refractive index between the ITO layer and the glass, the more distinctive is the total reflection at the interface. Thus, here economically produced thin glasses with high refractive index can be used in a very advantageous manner.